The Quad work well also for onshore and open sea crossover, but in such conditions a Kode also rips - maybe more in fact. If you want to replace your two smaller boards, something like the Kode 80 is definitely also worth considering for the conditions you describe.

As for the Quads, if it is the 69+79 you're replacing (ie you will keep the 91) then the Q76 is also an alternative. I think it will easily feel big enough to replace the JP 79 and in high wind it has a better chance of competing with that 69liter boards than the Q81 has. I think the stock fin setup will work well for most winds, but for truly nuking conditions you may wanna order a spare set of 13cm front fins. In fact, I used the Q66 in up to 45 knots mean wind this friday and it felt a bit stiff with the 15+11 setup I used. This was ridiculously windy stuff, not just your "normal" powered up 3.7 day, but even tough I'm not likely to sail something that windy any day soon I still took the plunge and ordered the 13cm fins for myself today. It will be interesting to see how they will make the board feel in more normal stuff too.